154 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
. Ads To doubly insure this, one of the lugs is marked to cor- 
respond with a particular support of the theodolite, the 
corresponding support being similarly marked. This 
SS arrangement further insures that even tho the center of 
the instrument does not correspond exactly with that of 
the point observed upon (for each monument), thus pre- 
venting a closing of the triangles within the accuracy of 
observation (that is, that the sum of the 3 angles of each 
triangle should equal 180°), the angles obtained will still 
) correspond with each other in successive observations 
within the limit of observational errors. 
For observing upon, and also for reference points in the 
PLATE base measurements, a spindle has been constructed for 
each plate. This spindle screws into a cup-like projec- 
tion fastened upon the plate for a center-mark. Each 
spindle is numbered to correspond with a particular monu- 
ment, the corresponding number being upon the plate. 
Up to a distance of about 200 feet these spindles make 
excellent objects to observe upon, provided there is a suit- 
able background. A background that can not be surpast 
is readily made by propping up behind the spindle the 
black iron cap which covers and protects the plate when 
TO MEASURE notinuse. When the distances are greater than about 
EARTH MOVEMENTS 200 feet, as is the case with the longer lines at Crystal 
1906 Springs, the best object to observe upon can be made by 
whittling the end of a lead pencil to fit into 
the cup and then wrapping the pencil with a 
little white cloth. In this case the back- 
ground should be the same as before. In 
any event, if tape measurements are con- 
templated, the spindles should be taken 
along, in order that they may be used as 
reference marks in those measurements. 
Fic. 45.— Diagram of bronze plate on monuments. Leveling record. — In the following tabu- 
Fia. 46.— Diagram of iron caps protecting plates. lation, S.W. p means the top of the bronze 
Fic. 47. — Diagram of spindles to be attached to plates. Plate on the southwest monument, along- 
side the spindle bowl or cup near the cen- 
ter. A corresponding point was taken on each monument to show the relative elevations 
to be retained for future reference. 
S.W.s is the top surface of the spindle hub, screwed into the socket, made for marking 
the center on the same plate. These points were taken to show differences of elevation of 
points used in the tape measures in the base-lines, and are of no value beyond this. 
In computing the elevations, the top of the southwest monument was arbitrarily taken as 
10 feet, and the other elevations are corrected to correspond with this datum plane. 
The spindle bow] is not in the center of the plate, owing to the position of the lugs, so that 
the point leveled upon representing the level of the plate is on the side of the spindle bowl 
next to the center of the plate. 
The level used was a Troughton and Simms dumpy level with compass attachment. 
13 “te 
Fie. 45 

W) 
mM 
O 
Fig. 46 
uw 

(4723 


[Elevations in feet. — Mean results.] 




First SECOND re Base-Line | DirrERENCES 
Measures. | MEASURES. : ELEVATIONS.| OF ELEVATION. 
OPW pot. cane e008 10.000 10.000 ri Gtesee Beeeere es 
S.W.s . 10.075 10.076 10.076 LO:07 Gia! ee ee 
Stake A. ee 7.465 7.462 7.464 7.464 — 2.612 
N.W.p. 8.006 8.004 Sina ee eee ee 
N.W.s 8.057 8.053 8.055 8.055 + 0.591 
Stake B 11.694 11.690 11.692 11.692 + 3.637 
S.E. p 11.389 11.390 PL SOO wel res | oop i ete ee 
S.E. s 11.455 11.456 11.456 11.456 — 0.236 
N.E. p 20.567 25.877 ZOOL 1 lees cea Tea Meeiemte ne we 
S.Wa6-) 2 5 SIS ee ee eee 10.076 — 1.380 


